1. Field of the Invention
The present invention relates to a cross connection (line setting) system for time-division multiplexed signals, and in particular to a cross connection system that includes a path switch in the exchange system for exchanging time-division multiplexed signals provided for an SDH (Synchronous Digital Hierarchy) network.
2. Related Arts
The SDH technique is an internationally standardized multiplex method that can be efficiently applied for telephone services and other services in transmission text data and picture image data.
In consonance with the progress that has been made in techniques concerning cross connection (line setting) apparatuses, a system where transmission lines for an entire network can be freely changed and improvements in the reliability of the transmission lines are demanded for recent transmission apparatuses.
Referring to FIG. 34, the connection setting for line signals VC-n can be freely changed by making a setup change. FIG. 34 shows the concept of a cross connection (line setting) apparatus that has the center role in an exchange system for exchanging time-division signals. As is shown in FIG. 34, a plurality of SDH interface frame signals, called STM-n (Synchronous Transmission Module Level n), are input. Line signals VC-n, which were multiplexed to obtain the STM-n frame signals, are extracted, and output terminals are determined to which VC-n signals are connected across the individual transmission lines. Then, the VC-n signals that have been set as described above are multiplexed again to obtain and output STM-n frame signals.
The STM-n frame signals are multiplexed transmission signals carried on the SDH network, and four different values, 0, 1, 4 and 16, are defined as the values for n at the synchronous transmission module level, the four values corresponding to the synchronous digital hierarchy levels, respectively. STM-1 is a base multiplex unit for the SDH network.
The line signals VC-n correspond to a box, called a virtual container, that is specified to multiplex various items of information as byte units. In addition to VC-4 that has a maximum bit rate of 150 Mbps and a retained information speed of 138 Mbps, there are VC-11, VC-12, VC-2 and VC3, which respectively have speeds corresponding to those for retained information.
Further, in FIG. 34, the connection setting for the VC-n signals can be freely changed by the alteration of the setup.
For a line that has only one output direction but has two input connections, as indicated in FIG. 34 by a solid line and a broken line, the same signal is passed along different routes from a cross connection apparatus at one location. In this case, the solid line indicates an active (ACT) line and the broken line indicates a standby (STB) line, and when deterioration of a signal carried on the ACT line is detected, the connection can be automatically changed to the STB line.
On the other hand, a line having outputs in two directions, relative to an input from one direction, is used to output the same signal in different directions in order to provide a redundant line connection. UNEQ (unequipped) denotes an empty time slot to which a line is not inserted.
As was previously described, to improve a system wherein the lines in a network can be freely changed, and to improve the reliability of the lines, an active and standby line redundancy system is employed for connecting the cross connection (line setting) apparatuses A and B, as is shown in FIG. 35.
In addition, as is shown in FIG. 36, in a RING application, transmission devices A through D having a limited function of line setting, called an Add/Drop multiplex function, are provided with individual line redundancy.
Since the conventional cross connection (line setting) apparatus employs the redundancy method performed across the multiplex STM-n lines, it can cope with the defect between the lines but can not handle a malfunction at the transmission device. Further, as active and standby transmission paths are provided on the same route, when both the active and standby transmission paths may be obstacles at the same time, the reliability of the cross connection apparatus has become low.
In the RING application, line redundancy can be applied only for a limited application, and it is difficult to constitute a line redundancy in a complicated network employing cross connection (line setting) apparatuses.